U.S. patent application number 15/432966 was filed with the patent office on 2017-11-30 for grid connection power conversion device and start-up control method therefor.
The applicant listed for this patent is Tabuchi Electric Co., Ltd.. Invention is credited to Hideki HIDAKA, Chienru LUNG.
Application Number | 20170346389 15/432966 |
Document ID | / |
Family ID | 59366115 |
Filed Date | 2017-11-30 |
United States Patent
Application |
20170346389 |
Kind Code |
A1 |
LUNG; Chienru ; et
al. |
November 30, 2017 |
Grid Connection Power Conversion Device and Start-Up Control Method
Therefor
Abstract
A grid connection power conversion device is provided in which
an amplitude of an output voltage from an inverter is increased
stepwise from 0, after start-up of the power conversion device, to
adjust the amplitude of the output voltage from the inverter to
match an amplitude of a commercial system voltage as detected, and
thereafter the inverter is connected to the commercial power system
to start grid-connected operation. This makes it possible to
prevent the voltage applied to a capacitor provided between the
inverter and the commercial power system from abruptly increasing
from 0V to the commercial system voltage at the start-up of the
power conversion device, thereby making it possible to reduce an
inrush current to the capacitor at the start-up without using an
inrush current prevention circuitry.
Inventors: |
LUNG; Chienru; (Osaka,
JP) ; HIDAKA; Hideki; (Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Tabuchi Electric Co., Ltd. |
Osaka |
|
JP |
|
|
Family ID: |
59366115 |
Appl. No.: |
15/432966 |
Filed: |
February 15, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02J 3/381 20130101;
H02M 1/32 20130101; H02M 3/04 20130101; Y02E 10/58 20130101; H02J
2300/26 20200101; H02M 1/126 20130101; H02M 1/36 20130101; Y02E
10/56 20130101; H02J 3/385 20130101; H02J 3/42 20130101; H02M
7/5387 20130101 |
International
Class: |
H02M 1/36 20070101
H02M001/36; H02J 3/38 20060101 H02J003/38; H02M 3/04 20060101
H02M003/04; H02M 7/5387 20070101 H02M007/5387 |
Foreign Application Data
Date |
Code |
Application Number |
May 27, 2016 |
JP |
2016-106060 |
Claims
1. A grid connection power conversion device for connecting a
distributed power supply to a commercial power system, the grid
connection power conversion device comprising: an inverter for
converting DC power based on power input from the distributed power
supply to AC power; a capacitor connected between the inverter and
the commercial power system; a commercial voltage amplitude
detection circuitry configured to detect an amplitude of a
commercial system voltage which is a voltage of the commercial
power system; an amplitude adjustment circuitry configured to
increase an amplitude of an output voltage from the inverter
stepwise from 0 to match the amplitude of the commercial system
voltage detected by the commercial voltage amplitude detection
circuitry; and a start-up control circuitry configured to control
the grid connection power conversion device so that after the
amplitude adjustment circuitry adjusts the amplitude of the output
voltage from the inverter, after the start-up of the grid
connection power conversion device, to match the amplitude of the
commercial system voltage detected by the commercial voltage
amplitude detection circuitry, the start-up control circuitry
connects the inverter to the commercial power system to start
grid-connected operation.
2. The grid connection power conversion device according to claim
1, further comprising: a commercial voltage phase angle detection
circuitry configured to detect a phase angle of the commercial
system voltage; and a phase angle adjustment circuitry configured
to adjust a phase angle of the output voltage from the inverter to
allow a phase difference between the phase angle of the output
voltage from the inverter and the phase angle of the commercial
system voltage detected by the commercial voltage phase angle
detection circuitry to have a value within a predetermined range,
wherein the start-up control circuitry controls the grid connection
power conversion device so that after the phase angle adjustment
circuitry adjusts the phase angle of the output voltage from the
inverter, after the start-up of the grid connection power
conversion device, so as to allow the phase difference to have a
value within a predetermined range, the start-up control circuitry
connects the inverter to the commercial power system to start the
grid-connected operation.
3. The grid connection power conversion device according to claim
1, further comprising a grid connection switch for switching the
connection of the distributed power supply to the commercial power
system between a connected state and a disconnected state, an
output current value detection circuitry configured to detect a
value of an output current from the inverter, a passing current
value estimation circuitry configured to estimate a value of a
capacitor-passing current which is a current flowing through the
capacitor, a system output current value calculation circuitry
configured to calculate a value of an output current from the grid
connection power conversion device based on the value of the output
current from the inverter detected by the output current value
detection circuitry and the value of the capacitor-passing current
estimated by the passing current value estimation circuitry, and a
disconnection/welding detection circuitry configured to detect
disconnection of the grid connection switch, before starting the
grid-connected operation, based on the value of the output current
from the grid connection power conversion device as calculated by
the system output current value calculation circuitry, with the
grid connection switch switched to the connected state, and also
configured to detect welding of the grid connection switch, before
starting the grid-connected operation, based on the value of the
output current from the grid connection power conversion device as
calculated by the system output current value calculation
circuitry, with the grid connection switch switched to the
disconnected state.
4. The grid connection power conversion device according to claim
3, further comprising an output voltage value detection circuitry
configured to detect a value of the output voltage from the
inverter, wherein the passing current value estimation circuitry
calculates the value of the capacitor-passing current based on the
value of the output voltage from the inverter as detected by the
output voltage value detection circuitry so as to estimate the
value of the capacitor-passing current.
5. The grid connection power conversion device according to claim
4, wherein the system output current value calculation circuitry
repeatedly calculates the value of the output current from the grid
connection power conversion device, with the grid connection switch
switched to the disconnected state, and the disconnection/welding
detection circuitry detects that welding occurs in the grid
connection switch if the values of the output current calculated
multiple times in the past are all equal to or more than a
predetermined threshold and continuously increase, and wherein if
the value of the output current calculated by the system output
current value calculation circuitry, with the grid connection
switch switched to the connected state, is less than the
predetermined threshold, the disconnection/welding detection
circuitry detects that disconnection occurs in the grid connection
switch.
6. The grid connection power conversion device according to claim
3, wherein the system output current value calculation circuitry
repeatedly calculates the value of the output current from the grid
connection power conversion device, with the grid connection switch
switched to the disconnected state, and the disconnection/welding
detection circuitry detects that welding occurs in the grid
connection switch if the values of the output current calculated
multiple times in the past are all equal to or more than a
predetermined threshold and continuously increase, and wherein if
the value of the output current calculated by the system output
current value calculation circuitry, with the grid connection
switch switched to the connected state, is less than the
predetermined threshold, the disconnection/welding detection
circuitry detects that disconnection occurs in the grid connection
switch.
7. A start-up control method for a grid connection power conversion
device comprising: an inverter for converting DC power based on
power input from a distributed power supply to AC power; and a
capacitor connected between the inverter and the commercial power
system, the start-up control method comprising the steps of:
increasing an amplitude of an output voltage from the inverter
stepwise from 0, after start-up of the grid connection power
conversion device, to adjust the amplitude of the output voltage
from the inverter to match an amplitude of a commercial system
voltage; and thereafter connecting the inverter to the commercial
power system to start grid-connected operation.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based on and claims the benefit of
priority of the prior Japanese Patent Application No. 2016-106060,
filed on May 27, 2016, the entire contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] The present invention relates to a grid connection power
conversion device (that is, power conversion device for grid
connection) applicable to a distributed power supply, and also
relates to a start-up control method for the power conversion
device.
2. Description of the Related Art
[0003] Conventionally, in order to use a distributed power supply
such as a solar cell and a fuel cell by connecting it to a
commercial power system, a power conversion device such as a power
conditioner is required to convert DC power generated by the
distributed power supply to AC power, which can be connected to the
commercial power system. If the distributed power supply is a solar
cell, the power conditioner performs Maximum Power Point Tracking
(hereafter referred to as MPPT) control to regulate an input
voltage from the solar cell so as to maximize (optimize) the output
voltage from the solar cell. More specifically, the power
conditioner comprises: a DC/DC converter for increasing or
decreasing the input voltage from the solar cell to a predetermined
voltage to increase or decrease the DC output voltage (DC bus
voltage V.sub.dc) within a certain range; and an inverter for
converting the DC power output from the DC/DC converter to AC
power.
[0004] The inverter comprises switching elements such as IGBTs
(Insulated Gate Bipolar Transistors) which are switched by a PWM
(Pulse Width Modulation) signal sent from a control unit of the
power conditioner. In order to remove the carrier of the PWM
signal, the power conditioner comprises an LC (inductor-capacitor)
filter for removing high frequency components, which is provided
between the inverter and the commercial power system. When the
inverter of the power conditioner is connected to the commercial
power system at start-up of the power conditioner, the voltage
applied to the capacitor of the LC filter abruptly increases from
0V to the commercial system voltage. Therefore, an inrush current
may flow in the capacitor to cause a contact of a grid connection
relay (that is, relay for grid connection) to be welded, and it is
considered that other circuitry elements (such as the switching
elements of the inverter) in the power conditioner may be
broken.
[0005] Thus, in order to reduce an inrush current to the capacitor,
a power conversion device is known which has an inrush current
prevention circuitry formed by a parallel circuitry of a switch and
a resistor and connected in series to the capacitor, and which is
designed to detect a failure of the switch in the inrush current
prevention circuitry, as described in Japanese Laid-open Patent
Publication 2015-027146. Further, in this kind of power conversion
device, as described in Japanese Laid-open Patent Publication
2014-064415, it is known to output a control signal to a relay
(second switch 16) for isolated power system to bring the relay to
an open state, and also output a test voltage V3 from an inverter
(power conversion unit 13) so as to detect welding of a contact of
the relay for isolated power system (welding between internal
terminals of the second switch 16) based on a voltage value of a
terminal for isolated operation as then detected by a voltage
detection unit of the isolated power system. Furthermore, in this
kind of power conversion device, as described in Japanese Laid-open
Patent Publication 2005-318704, it is known to use a change rate of
an applied voltage to a capacitor, without adding an exclusive
current detection sensor, to detect a short circuit of a switching
element of a power converting section when both upper and lower
arms of the switching element are turned on at the same time due to
conduction failure or control abnormality of the switching
element.
[0006] However, these power conversion devices have the following
problems. In the power conversion device (power conditioner)
described in Japanese Laid-open Patent Publication 2015-027146, it
is necessary to newly provide an inrush current prevention
circuitry formed by a parallel switch and a resistor and the like,
and therefore the manufacturing cost of the power conversion device
increases. Further, in the power conversion device described in
Japanese Laid-open Patent Publication 2014-064415, it is possible
to detect welding of a contact of a relay for isolated power
system, but it is not possible to reduce an inrush current to the
capacitor of the LC filter (provided between the inverter and the
commercial power system). Furthermore, in the power conversion
device described in Japanese Laid-open Patent Publication
2005-318704, it is possible to detect a short circuit of the
switching element, but it is not possible to reduce an inrush
current to the capacitor of the LC filter.
BRIEF SUMMARY OF THE INVENTION
[0007] An object of the present invention is to solve the problems
described above, and to provide a grid connection power conversion
device and a start-up control method for the grid connection power
conversion device which make it possible to reduce an inrush
current to a capacitor connected between an inverter and a
commercial power system at start-up of the power conversion device
without using an inrush current prevention circuitry.
[0008] According to a first aspect of the present invention, this
object is achieved by a grid connection power conversion device for
connecting a distributed power supply to a commercial power system,
the grid connection power conversion device comprising: an inverter
for converting DC power based on power input from the distributed
power supply to AC power; a capacitor connected between the
inverter and the commercial power system; a commercial voltage
amplitude detection circuitry configured to detect an amplitude of
a commercial system voltage which is a voltage of the commercial
power system; an amplitude adjustment circuitry configured to
increase an amplitude of an output voltage from the inverter
stepwise from 0 to match the amplitude of the commercial system
voltage detected by the commercial voltage amplitude detection
circuitry; and a start-up control circuitry configured to control
the grid connection power conversion device so that after the
amplitude adjustment circuitry adjusts the amplitude of the output
voltage from the inverter, after the start-up of the grid
connection power conversion device, to match the amplitude of the
commercial system voltage detected by the commercial voltage
amplitude detection circuitry, the start-up control circuitry
connects the inverter to the commercial power system to start
grid-connected operation.
[0009] It is preferred that the grid connection power conversion
device further comprises a commercial voltage phase angle detection
circuitry configured to detect a phase angle of the commercial
system voltage, and a phase angle adjustment circuitry configured
to adjust a phase angle of the output voltage from the inverter to
allow a phase difference between the phase angle of the output
voltage from the inverter and the phase angle of the commercial
system voltage detected by the commercial voltage phase angle
detection circuitry to have a value within a predetermined range,
therein the start-up control circuitry controls the grid connection
power conversion device so that after the phase angle adjustment
circuitry adjusts the phase angle of the output voltage from the
inverter, after the start-up of the grid connection power
conversion device, so as to allow the phase difference to have a
value within a predetermined range, the start-up control circuitry
connects the inverter to the commercial power system to start the
grid-connected operation.
[0010] It is preferred that the grid connection power conversion
device further comprises a grid connection switch for switching the
connection of the distributed power supply to the commercial power
system between a connected state and a disconnected state, an
output current value detection circuitry configured to detect a
value of an output current from the inverter, a passing current
value estimation circuitry configured to estimate a value of a
capacitor-passing current which is a current flowing through the
capacitor, a system output current value calculation circuitry
configured to calculate a value of an output current from the grid
connection power conversion device based on the value of the output
current from the inverter detected by the output current value
detection circuitry and the value of the capacitor-passing current
estimated by the passing current value estimation circuitry, and a
disconnection/welding detection circuitry configured to detect
disconnection of the grid connection switch, before starting the
grid-connected operation, based on the value of the output current
from the grid connection power conversion device as calculated by
the system output current value calculation circuitry, with the
grid connection switch switched to the connected state, and also
configured to detect welding of the grid connection switch, before
starting the grid-connected operation, based on the value of the
output current from the grid connection power conversion device as
calculated by the system output current value calculation
circuitry, with the grid connection switch switched to the
disconnected state.
[0011] It is preferred that the grid connection power conversion
device further comprises an output voltage value detection
circuitry configured to detect a value of the output voltage from
the inverter, therein the passing current value estimation
circuitry calculates the value of the capacitor-passing current
based on the value of the output voltage from the inverter as
detected by the output voltage value detection circuitry so as to
estimate the value of the capacitor-passing current.
[0012] In the grid connection power conversion device, it is
preferred that the system output current value calculation
circuitry repeatedly calculates the value of the output current
from the grid connection power conversion device, with the grid
connection switch switched to the disconnected state, and the
disconnection/welding detection circuitry detects that welding
occurs in the grid connection switch if the values of the output
current calculated multiple times in the past are all equal to or
more than a predetermined threshold and continuously increase,
therein if the value of the output current calculated by the system
output current value calculation circuitry, with the grid
connection switch switched to the connected state, is less than the
predetermined threshold, the disconnection/welding detection
circuitry detects that disconnection occurs in the grid connection
switch.
[0013] According to a second aspect of the present invention, the
above object is achieved by a start-up control method for a grid
connection power conversion device comprising: an inverter for
converting DC power based on power input from a distributed power
supply to AC power; and a capacitor connected between the inverter
and the commercial power system, the start-up control method
comprising the steps of: increasing an amplitude of an output
voltage from the inverter stepwise from 0, after start-up of the
grid connection power conversion device, to adjust the amplitude of
the output voltage from the inverter to match an amplitude of a
commercial system voltage; and thereafter connecting the inverter
to the commercial power system to start grid-connected
operation.
[0014] While the novel features of the present invention are set
forth in the appended claims, the present invention will be better
understood from the following detailed description taken in
conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention will be described hereinafter with
reference to the annexed drawings. It is to be noted that the
drawings are shown for the purpose of illustrating the technical
concepts of the present invention or embodiments thereof,
wherein:
[0016] FIG. 1 is a schematic circuit diagram, partially in block
form, showing an outline of a solar photovoltaic power generation
system including a power conditioner (grid connection power
conversion device) according to an exemplary embodiment of the
present invention;
[0017] FIG. 2 is a schematic block diagram showing an outline of
control of the power conditioner;
[0018] FIG. 3 is a schematic block diagram showing an amplitude
adjustment control of an output voltage from the inverter, which
the power conditioner performs at or after its start-up;
[0019] FIG. 4 is a flow chart of a control process of the power
conditioner at its start-up;
[0020] FIG. 5 is a schematic chart showing an on/off switching of
grid connection relays of the power conditioner in a
disconnection/welding detection process of each grid connection
relay;
[0021] FIG. 6 is a graph showing a method to determine
disconnection/welding of the grid connection relays;
[0022] FIG. 7 is a graph showing results of an experiment conducted
at start-up of the power conditioner with one of the grid
connection relays welded; and
[0023] FIG. 8 is a schematic view showing, in the left side, a
relay circuitry of grid connection relays in the power conditioner
according to the exemplary embodiment, and showing, in the right
side, a relay circuitry of grid connection relays and protection
relays in a power conditioner according to Modified Example 1 of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Hereinafter, a grid connection power conversion device and a
start-up control method for the grid connection power conversion
device according to an exemplary embodiment of the present
invention will be described with reference to the drawings. The
present embodiment shows an example where the claimed grid
connection power conversion device is a power conditioner. FIG. 1
is a schematic circuit diagram, partially in block form, showing an
outline of a solar photovoltaic power generation system 10
including a power conditioner (grid connection power conversion
device) 1 according to the exemplary embodiment. The solar
photovoltaic power generation system 10 comprises: a solar cell 2
as a distributed power supply; and the power conditioner 1 for
converting DC power generated by the solar cell 2 to AC power, and
can be connected (grid-connected) to a commercial power system
(commercial power grid) 3. The power conditioner 1 comprises: a
DC/DC converter 4; an electrolytic capacitor C.sub.dc for smoothing
DC bus voltage; an inverter 5; an LC (inductor-capacitor) filter 6;
a control circuitry 7; and grid connection relays (that is, relays
for grid connection) S.sub.u and S.sub.w (claimed "grid connection
switch").
[0025] The DC/DC converter 4 performs Maximum Power Point Tracking
(hereafter referred to as MPPT) control of the solar cell 2 to
adjust an input voltage from the solar cell 2 to maximize
(optimize) an output power from the solar cell 2. More
specifically, the DC/DC converter 4 increases or decreases the
input voltage from the solar cell 2 to a predetermined voltage to
increase or decrease the DC output voltage (DC bus voltage
V.sub.dc) within a certain range. The inverter 5 converts the DC
power based on the power output from the DC/DC converter 4 to AC
power. The inverter 5 comprises switching elements SW1 to SW4
formed by IGBTs (Insulated Gate Bipolar Transistors) which are
switched by a PWM (Pulse Width Modulation) signal sent from a
control unit of the power conditioner 1. On the other hand, the LC
filter 6 comprises two inductors (reactors) L.sub.f each connected
in series with each of two power supply lines, and a capacitor
C.sub.f connected between the two power supply lines to remove
harmonic (high frequency) components (mainly the carrier frequency
of the PWM signal) from the AC voltage output from the inverter 5.
In FIG. 1, R.sub.f and R.sub.c represent an inner resistance of the
inductor L.sub.f and an inner resistance of the capacitor C.sub.f,
respectively. The capacitor C.sub.f of the LC filter 6 corresponds
to the claimed "capacitor connected between the inverter and the
commercial power system".
[0026] The control circuitry 7 is formed by a so-called
microcomputer, and comprises circuitries including a commercial
voltage amplitude detection circuitry 11, an amplitude adjustment
circuitry 12, a start-up control circuitry 13, a PLL (Phase Locked
Loop) circuitry (synchronous circuitry) 14, a phase angle
adjustment circuitry 15, a disconnection/welding detection
circuitry 16, an output voltage amplitude detection circuitry 17,
an output current value detection circuitry 18, a passing current
value estimation circuitry 19, a system output current value
calculation circuitry 20 and an output voltage value detection
circuitry 21. These circuitries 11 to 21 are formed by basic
function blocks of the microcomputer. The PLL circuitry 14
corresponds to the claimed commercial voltage phase angle detection
circuitry.
[0027] The commercial voltage amplitude detection circuitry 11 of
the control circuitry 7 is configured to detect an amplitude of a
commercial system voltage e.sub.uw (line-to-line voltage between
the U-phase and the W-phase). The amplitude adjustment circuitry 12
is configured to increase an amplitude of an output voltage
e.sub.inv from the inverter 5 stepwise from 0 to adjust the
amplitude of the output voltage from the inverter 5 to match or
follow the amplitude of the commercial system voltage e.sub.uw
detected by the commercial voltage amplitude detection circuitry
11. The start-up control circuitry 13 is configured to start up and
control the hybrid power generation and storage system 1 so that
after the amplitude adjustment circuitry 12 adjusts the amplitude
of the output voltage e.sub.inv from the inverter 5, after the
start-up of the power conditioner 1, to match the amplitude of the
commercial system voltage e.sub.uw detected by the commercial
voltage amplitude detection circuitry 11, the start-up control
circuitry 13 connects the inverter 5 to the commercial power system
3 so as to allow the power conditioner 1 to smoothly shift to
grid-connected operation control (that is, to smoothly start
grid-connected operation).
[0028] The PLL circuitry 14 is configured to output a signal
synchronized with a reference input signal, and is used to generate
a voltage signal synchronized with a phase angle .theta..sub.uw of
the commercial system e.sub.uw. This PLL circuitry 14 is also used
to detect the phase angle .theta..sub.uw of the commercial system
voltage e.sub.uw as will be described later. The phase angle
adjustment circuitry 15 is configured to adjust a phase angle of
the output voltage e.sub.inv from the inverter 5 after (immediately
after) the start-up of the power conditioner 1 to allow a phase
difference between the phase angle of the output voltage e.sub.inv
from the inverter 5 and the phase angle .theta..sub.uw of the
commercial system voltage e.sub.uw detected by the PLL circuitry 14
to have a certain value (.DELTA..phi. described later). The
disconnection/welding detection circuitry 16 is configured to
detect disconnection and welding of the grid connection relays
S.sub.u and S.sub.w. The detection process performed by the
disconnection/welding detection circuitry 16 will be described in
detail later. The output voltage amplitude detection circuitry 17
detects an amplitude of the output voltage e.sub.inv from the
inverter 5.
[0029] The output current value detection circuitry 18 detects a
value of an output current (hereafter referred to as "inverter
output current") i.sub.inv from the inverter 5. The passing current
value estimation circuitry 19 estimates a value of a current
(capacitor-passing current) i.sub.c flowing through the capacitor
C.sub.f. The system output current value calculation circuitry 20
calculates a value of an output current i.sub.sp from the power
conditioner 1 based on the value of the inverter output current
i.sub.inv detected by the output current value detection circuitry
18 and the value of the capacitor-passing current i.sub.c estimated
by the passing current value estimation circuitry 19. The output
voltage value detection circuitry 21 detects a value of the output
voltage e.sub.inv from the inverter 5.
[0030] The grid connection relays S.sub.u and S.sub.w (claimed
"grid connection switch") serve as a switch for switching the
connection of the solar cell 2 (and the power conditioner 1) to the
commercial power system 3 between a connected state and a
disconnected state. The commercial power system 3 includes a
commercial system power supply 8 and a commercial system (grid)
impedance. In FIG. 1, R.sub.Grid and L.sub.Grid represent a
resistance and an inductance (inductive reactance) of the
commercial system impedance, respectively, while R.sub.uw
represents an AC load connected to the commercial power system
3.
[0031] FIG. 2 is a schematic block diagram showing an outline of
control of the control circuitry 7 of the power conditioner 1. FIG.
2 shows a grid-connected operation control section and a start-up
control unit 35. The control circuitry 7 of the power conditioner 1
controls an output current from the inverter 5 for active and
reactive components in grid-connected operation of the power
conditioner 1 (when a switch S.sub.Grid is in position 0), and
controls an AC output voltage from the inverter 5 at start-up of
the power conditioner 1 (when the switch S.sub.Grid is in position
1). As shown in FIG. 2, the grid-connected operation control
section comprises an active component (sin(.theta..sub.uw))
generation unit 31, a reactive component (cos(.theta..sub.uw))
generation unit 32 and an inverter output current control unit
33.
[0032] In the grid-connected operation with the switch S.sub.Grid
in position 0, the active component generation unit 31 multiplies
an output current command value I*.sub.p for the active component
and a sine value sin(.theta..sub.uw) of the phase angle
.theta..sub.uw of the commercial system voltage e.sub.uw output
from the PLL circuitry 14 to generate an instantaneous output value
of the output current command value I*.sub.p for the active
component. On the other hand, the reactive component generation
unit 32 multiplies an output current command value I*.sub.q for the
reactive component and a cosine value cos(.theta..sub.uw) of the
phase angle .theta..sub.uw of the commercial system voltage
e.sub.uw output from the PLL circuitry 14 to generate an
instantaneous output value of the output current command value
I*.sub.p for the reactive component. Note that as shown in FIG. 2,
f.sub.0 represents a fundamental frequency input to the PLL
circuitry 14, while f.sub.uw represents a commercial system
frequency obtained by the PLL circuitry 14.
[0033] The instantaneous output value from the active component
generation unit 31 and the instantaneous output value from the
reactive component generation unit 32 are summed at a summing point
SP1 to produce an output current command value i*.sub.inv for the
inverter 5. The output current command value i*.sub.inv and an
output current i.sub.inv from the inverter 5 as a feedback value
are sent to the inverter output current control unit 33. The
inverter output current control unit 33 performs a feedback control
such that the value of the output current i.sub.inv from the
inverter 5 matches or follows the output current command value
i*.sub.inv, so as to calculate a duty ratio d.sub.a of PWM control
for the grid-connected operation. The duty ratio d.sub.a is input
to the PWM control unit 34. Based on the input duty ratio d.sub.a,
the PWM control unit 34 generates a PWM signal having a pulse width
corresponding to the duty ratio d.sub.a. This PWM signal is used to
control the on/off of each of the switches SW1, SW2, SW3 and SW4 of
the inverter 5.
[0034] Referring again to FIG. 2, when the switch S.sub.Grid is in
position 1, the control circuitry 7 controls the AC output voltage
from the inverter 5 at start-up of the power conditioner 1. More
specifically, the start-up control unit 35 of the control circuitry
7 uses the following Equation (1) to calculate a duty ratio d.sub.b
of PWM control for the start-up. In Equation (1), E*.sub.inv.max
represents a command value of the maximum value of the output
voltage (amplitude of the output voltage) from the inverter 5,
while .DELTA..phi. is a threshold of the phase angle which is newly
introduced to provide a difference between the phase angle of the
output voltage from the inverter 5 and the phase angle
.theta..sub.uw of the commercial system voltage e.sub.uw. Further,
V.sub.dc in Equation (1) represents a value of the DC bus voltage
detected by the control circuitry 7. The command value
E*.sub.inv.max of the maximum value of the output voltage from the
inverter 5 is calculated in a process of amplitude adjustment
control of the output voltage from the inverter 5 as shown in FIG.
3. The phase angle .theta..sub.uw of the commercial system obtained
by the PLL circuitry 14 is used as the phase angle .theta..sub.uw
in Equation (1). Such use of the phase angle .theta..sub.uw of the
commercial system voltage e.sub.uw obtained by the PLL circuitry 14
makes it possible to provide a slight phase difference
(.DELTA..phi.) between the phase angle of the output voltage from
the inverter 5 and the phase angle .theta. of the commercial system
voltage e.sub.uw.
d b = E inv ma x * sin ( .theta. uw + .DELTA..phi. ) V d c ( 1 )
##EQU00001##
[0035] Based on the duty ratio d.sub.b calculated by the start-up
control unit 35 of the control circuitry 7, the PWM control unit 34
generates a PWM signal having a pulse width corresponding to the
duty ratio d.sub.b. This PWM signal is used to control the on/off
of each of the switches SW1, SW2, SW3 and SW4 of the inverter 5.
Thus, at start-up of the power conditioner 1, the amplitude of the
output voltage from the inverter 5 substantially matches or follows
the command value E*.sub.inv.max of the maximum value (amplitude)
of the output voltage from the inverter 5. The power conditioner 1
of the present exemplary embodiment uses a start-up control method
in which in order to prevent or reduce an inrush current to a
capacitor C.sub.f on the output side of the inverter 5 at its
start-up, the power conditioner 1 adjusts the amplitude of the
output voltage from the inverter 5 to match or follow the amplitude
of the commercial system voltage before shifting to the
grid-connected operation, and thereafter smoothly shifts to the
grid-connected operation control.
[0036] FIG. 3 is a schematic block diagram showing an amplitude
adjustment control block of an output voltage from the inverter 5,
which the control circuitry 7 (mainly the amplitude adjustment
circuitry 12) performs in a feedback control of the output voltage
from the inverter 5 at or after its start-up. More specifically,
this amplitude adjustment control block controls to adjust the
maximum value E.sub.inv.max of the output voltage (amplitude of the
output voltage) from the inverter 5 to match a maximum value
E.sub.uw.max of the commercial system voltage e.sub.uw (amplitude
of the commercial system voltage e.sub.uw). Note that in the
following description, the output voltage from the inverter 5 will
be referred to as inverter output voltage.
[0037] In FIG. 3, E.sub.uw.max is the maximum value (amplitude) of
the commercial system voltage e.sub.uw, and E.sub.inv.max is the
maximum value (amplitude) of the inverter output voltage, while
E*.sub.inv.max(k-1) is a command value of the maximum value
E.sub.inv.max of the inverter output voltage in a sampling period
preceding (immediately preceding) the k-th sampling period. Note
that (k) represents a value in the k-th sampling period which means
the current (present) value, while (k-1) represents a value in the
sampling period preceding the k-th sampling period, that is, a
value preceding the current value. Also note that the sampling
period in the present exemplary embodiment is an inverse of a
switching frequency.
[0038] First, the control circuitry 7 (mainly the amplitude
adjustment circuitry 12) of the power conditioner 1 sets, to 0,
both the initial value of the maximum value E.sub.inv.max of the
inverter output voltage e.sub.inv and the initial value of the
preceding command value E*.sub.inv.max(k-1) of the maximum value
E.sub.inv.max of the inverter output voltage e.sub.inv. Then, the
control circuitry 7 subtracts, at a summing point SP2, the maximum
value E.sub.inv.max (current (present) maximum value expressible as
E.sub.inv.max(k) as well) of the inverter output voltage e.sub.inv
in a specific sampling period (k-th or current sampling period)
from the maximum value (amplitude) E.sub.uw.max of the commercial
system voltage e.sub.uw (as detected by the commercial voltage
amplitude detection circuitry 11) in the same specific sampling
period to obtain a difference value between the two values. As
shown in FIG. 3, the control circuitry 7 comprises a proportional
control unit (K.sub.p) 37 which multiplies the difference value by
a proportional gain K.sub.p to output an output value (product of
the multiplication) to a summing point SP3. Here, a positive value
(such as 0.01) which is much lower than 1 is used as the
proportional gain K.sub.p.
[0039] Next, the control circuitry 7 sums the output value from the
proportional control unit 37 at the summing point SP3 with the
command value E*.sub.inv.max(k-1) of the maximum value
E.sub.inv.max of the inverter output voltage e.sub.inv in the
(k-1)th sampling period (such command value E*.sub.inv.max(k-1) may
hereafter be referred to as preceding command value
E*.sub.inv.max(k-1) as well) preceding the specific sampling period
(k-th or current sampling period) so as to calculate a command
value E*.sub.inv.max(k) of the maximum value E.sub.inv.max of the
inverter output voltage e.sub.inv as the current command value
E*.sub.inv.max(k). The thus calculated current command value
E*.sub.inv.max(k) is used as the command value of the maximum value
of the inverter output voltage in the above Equation (1).
[0040] The use of a positive value much lower than 1 as the
proportional gain K.sub.p as described above can increase the
preceding command value E*.sub.inv.max(k-1) of the maximum value
E.sub.inv.max of the inverter output voltage e.sub.inv stepwise
from 0 by repeating the summation, making it possible to adjust the
maximum value (amplitude) E.sub.inv.max of the inverter output
voltage e.sub.inv to gradually match or follow the maximum value
(amplitude) E.sub.uw.max of the commercial system voltage e.sub.uw.
By using such a method of setting both the initial values of
E.sub.inv.max and E*.sub.inv.max(k-1) to 0 and then stepwise
adjusting the maximum value (amplitude) E.sub.inv.max of the
inverter output voltage e.sub.inv to gradually match maximum value
(amplitude) E.sub.uw.max of the commercial system voltage e.sub.uw
as described above, a soft start effect (to start the operation or
control with a soft start) at start-up of the power conditioner 1
can be obtained.
[0041] The following Equations (2) to (4) are used to calculate the
maximum value E.sub.inv.max of the inverter output voltage
e.sub.inv and the maximum value E.sub.uw.max of the commercial
system voltage e.sub.inv. In these Equations, T.sub.uw and f.sub.uw
are the period and frequency of the commercial system voltage
e.sub.uw, respectively.
E inv ma x = 4 T uw .intg. 0 T uw 2 e inv 2 ( t ) dt ( 2 ) E uw ma
x = 4 T uw .intg. 0 T uw 2 e uw 2 ( t ) dt ( 3 ) T uw = 1 f uw ( 4
) ##EQU00002##
[0042] According to the present exemplary embodiment, the power
conditioner 1 can be smoothly started up, because at or after
start-up of the power conditioner 1, the method of amplitude
adjustment control shown in FIG. 3 is used to adjust the maximum
value (amplitude) E.sub.inv.max of the inverter output voltage
e.sub.inv, and the phase angle adjustment circuitry 15 of the
control circuitry 7 adjusts the phase angle of the inverter output
voltage, using the above Equation (1), to allow a phase difference
between the phase angle of the inverter output voltage and the
phase angle .theta..sub.uw of the commercial system voltage
e.sub.uw detected by the PLL circuitry 14 to have a certain value
(.DELTA..phi.). Here, the phrase "can be smoothly started up" means
that an abrupt increase in the maximum value (amplitude)
E.sub.inv.max of the output voltage from the inverter 5 can be
avoided at start-up of the power conditioner 1.
[0043] Next, referring to the flow chart of FIG. 4, a flow of a
control process of the power conditioner 1 according to the present
exemplary embodiment at its start-up will be described. Before
connection to the commercial power system (grid) 3, the control
circuitry 7 (mainly the start-up control circuitry 13) of the power
conditioner 1 switches the switch S.sub.GRID to position 1, and
turns off each of the grid connection relays S.sub.u, S.sub.w to
perform a control process shown in FIG. 3 so as to adjust the
amplitude of the inverter output voltage e.sub.inv to match the
amplitude of the commercial system voltage e.sub.uw (step S1). When
a predetermined time from the start of the amplitude adjustment
process of step S1 above has elapsed (YES in step S2), and the
amplitude of the inverter output voltage e.sub.inv reaches the
amplitude of the commercial system voltage e.sub.uw, the control
circuitry 7 (mainly the start-up control circuitry 13 and the
disconnection/welding detection circuitry 16) of the power
conditioner 1 turns the grid connection relays S.sub.u, S.sub.w on
and off as shown in FIG. 5 below to detect disconnection and
welding of the grid connection relays S.sub.u and S.sub.w (step S3
in FIG. 4).
[0044] This will be described in detail below with reference to
FIG. 5, which is a schematic chart showing an on/off switching of
the grid connection relays S.sub.u, S.sub.w in a
disconnection/welding detection process of each of the grid
connection relays S.sub.u, S.sub.w. Referring to FIG. 5, in period
(1), with both grid connection relays S.sub.u, S.sub.w turned off,
the control circuitry 7 detects whether both grid connection relays
S.sub.u, S.sub.w are welded. Further, in period (2), with the grid
connection relay S.sub.w kept turned off, the control circuitry 7
turns only the grid connection relay S.sub.u on, and detects
whether the grid connection relay S.sub.w is welded. Further, in
period (3), when a predetermined time T.sub.dly elapses from the
end of period (2), the control circuitry 7 turns the grid
connection relay S.sub.u off and the grid connection relay S.sub.w
on, and detects whether the grid connection relay S.sub.u is
welded. Furthermore, in period (4), when a predetermined time
T.sub.dly elapses from the end of period (3), the control circuitry
7 detects, with both grid connection relays S.sub.u, S.sub.w turned
on, whether the grid connection relay S.sub.u or the grid
connection relay S.sub.w is disconnected.
[0045] Next, a specific disconnection/welding detection process of
the grid connection relays S.sub.u, S.sub.w will be described.
First, based on a value of the inverter output voltage e.sub.inv
detected by the output voltage value detection circuitry 21, the
control circuitry 7 (its passing current value estimation circuitry
19) estimates (calculates) a value of a current i.sub.c (hereafter
referred to as capacitor-passing current) flowing through the
capacitor C.sub.f, using the following Equation (5). Subsequently,
the control circuitry 7 (its system output current value
calculation circuitry 20) calculates a value of an output current
i.sub.sp from the power conditioner 1, using the following Equation
(6), based on the value of the inverter output current i.sub.inv
detected by the output current value detection circuitry 18 and the
value of the capacitor-passing current i.sub.c calculated by using
Equation (5).
[0046] For detection of whether both grid connection relays
S.sub.u, S.sub.w are welded in period (1) in FIG. 5, the control
circuitry 7 (its disconnection/welding detection circuitry 16)
determines that welding occurs in both grid connection relays
S.sub.u, S.sub.w, if an absolute value |i.sub.sp| of the output
current i.sub.sp calculated by Equation (6) is equal to or more
than a threshold I.sub.chk calculated by the following Equation
(7). On the other hand, for detection of whether only one of the
grid connection relays S.sub.u, S.sub.w (which is turned off) is
welded in periods (2) and (3) in FIG. 5, the control circuitry 7
(its system output current value calculation circuitry 20)
repeatedly calculates the absolute value |i.sub.sp| of the output
current i.sub.sp, and the control circuitry 7 (its
disconnection/welding detection circuitry 16) determines that
welding occurs in only one of the grid connection relays S.sub.u,
S.sub.w, if the absolute values |i.sub.sp| of the output current
i.sub.sp calculated multiple times in the past such as three times
in the past are all equal to or more than the threshold I.sub.chk
calculated by the following Equation (7), and if such calculated
absolute values |i.sub.sp| continuously increase as shown in FIG.
6. Then, if, in period (1), (2) or (3), the control circuitry 7
determines that welding occurs in the grid connection relay S.sub.u
and/or the grid connection relay S.sub.w (NO in step S3 in FIG. 4),
the control circuitry 7 stops the operation of the power
conditioner 1 (step S4 in FIG. 4). Note that a detailed description
of the following Equation (5) is omitted because it is well known
as a transfer function of an RC (resistor-capacitor) series circuit
in the field of control engineering.
i c = sC f sR c C f + 1 e inv ( 5 ) i sp = i inv - i c ( 6 ) I chk
= 2 .pi. f uw C f E uw ma x x ( 7 ) ##EQU00003##
[0047] The numerator
(2.pi.f.sub.uwC.sub.fE.sub.uw.max=(.omega..sub.uwC.sub.fE.sub.uw.max))
of the above Equation (7) to calculate the threshold I.sub.chk
shows a maximum value of the current (capacitor-passing current)
i.sub.c flowing through the capacitor C.sub.f when the grid
connection relays S.sub.u, S.sub.w are turned on. Note that x in
Equation (7) is a factor to set the threshold I.sub.chk for
detection level. It is considered that the value of this factor
should be set to x>1.
[0048] Now, the reason why, in periods (2) and (3) in FIG. 5, the
continuous increase of the absolute values |i.sub.sp| of the output
current i.sub.sp calculated multiple times in the past is used as a
condition to determine the welding of the grid connection relays
S.sub.u, S.sub.w is as follows. If, in periods (2) and (3) in FIG.
5, one of the grid connection relays S.sub.u, S.sub.w (which is
turned off for welding detection) is welded, the inverter 5 is
connected to the commercial power system 3 when the other of the
grid connection relays is turned on, and therefore, an inrush
current to the capacitor C.sub.f occurs (at the moment when the
inverter 5 is connected to the commercial power system 3) due to
the phase difference .DELTA..phi. between the inverter output
voltage e.sub.inv and the commercial system voltage e.sub.uw, and
thus, an abrupt increase in the current value of the output current
i.sub.sp from the power conditioner 1 occurs mainly due to the
inrush current to the capacitor C.sub.f.
[0049] In contrast, the reason why, in period (1) in FIG. 5, the
continuous increase of the absolute values |i.sub.sp| of the output
current i.sub.sp is not used as a condition to determine the
occurrence of welding of the grid connection relays S.sub.u,
S.sub.w while only that the absolute value |i.sub.sp| of the output
current i.sub.sp is equal to or more than the threshold I.sub.chk
is used as a condition to determine the occurrence of welding of
the grid connection relays S.sub.u, S.sub.w is as follows. If
welding occurs in both of the grid connection relays S.sub.u,
S.sub.w, the inverter 5 is already connected to the commercial
power system 3 before period (1) begins, and therefore, an abrupt
increase in the current value of the output current i.sub.sp mainly
due to the inrush current to the capacitor C.sub.f does not occur
at the moment of the beginning of period (1).
[0050] Referring back to the flow chart of FIG. 4, if, in period
(4) in FIG. 5, the output current i.sub.sp from the power
conditioner 1 as calculated by the above Equations (5) and (6) is
less than the threshold I.sub.chk calculated by Equation (7), the
control circuitry 7 (its disconnection/welding detection circuitry
16) determines that disconnection occurs in at least one of the
grid connection relays S.sub.u, S.sub.w (NO in step S3 in FIG. 4),
and stops the power conditioner 1 (step S4 in FIG. 4).
[0051] A summary of the disconnection/welding detection process
described above is that before starting the grid-connected
operation, the control circuitry 7 (its disconnection/welding
detection circuitry 16) detects disconnection of the grid
connection relays S.sub.u, S.sub.w based on a value of the output
current i.sub.sp calculated by the system output current value
calculation circuitry 20, with the grid connection relays S.sub.u,
S.sub.w turned on (that is, when the grid connection switch is
switched to a connected state), and also detects welding of the
grid connection relays S.sub.u, S.sub.w based on the value of the
output current i.sub.sp calculated by the system output current
value calculation circuitry 20, with the grid connection relays
S.sub.u, S.sub.w turned off (that is, when the grid connection
switch is switched to a disconnected state).
[0052] Note here that a power conversion device to detect welding
of an isolated power system relay is described in Japanese
Laid-open Patent Publication 2014-064415 as discussed in the
Description of the Related Art. However, although the power
conversion device described in Japanese Laid-open Patent
Publication 2014-064415 can detect welding of the isolated power
system relay, it cannot detect disconnection of the isolated power
system relay. Further, it may be possible to use the technology of
detecting an abnormality of a switching element as described in
Japanese Laid-open Patent Publication 2005-318704 to detect welding
of the grid connection relays S.sub.u, S.sub.w. However, in the
case of the method as described in this Japanese Laid-open Patent
Publication 2005-318704 to detect short circuit (welding) of a
switching element (corresponding to the grid connection relays
S.sub.u, S.sub.w) of a power conversion unit (inverter) based on a
change rate of an applied voltage to an input capacitor
(corresponding to the capacitor C.sub.f in FIG. 1), it is very
likely that if a set value of the change rate of the applied
voltage to the capacitor (hereafter referred to as "capacitor
voltage") which serves as a threshold for short circuit detection
is high, the switching element may be broken due to an over current
at the moment of the beginning of the short circuit. Conversely, if
the set value of the change rate of the capacitor voltage serving
as the threshold is low, it becomes very likely that erroneous
detection may occur, for example, due to noise.
[0053] Referring back to the flow chart of FIG. 4, if, in the
disconnection/welding detection process of step S3, the control
circuitry 7 does not detect welding or disconnection of the grid
connection relays S.sub.u, S.sub.w (YES in step S3), the control
circuitry 7 (mainly the start-up control circuitry 13) switches the
switch S.sub.Grid to position 0 and turns the grid connection
relays S.sub.u, S.sub.w on to start grid-connected operation of the
power conditioner 1.
[0054] The start-up control method for the power condition 1
according to the present exemplary embodiment described above has
two features. The first feature is to use a feedback control to
adjust the amplitude E.sub.inv.max of the inverter output voltage
to gradually match the amplitude E.sub.uw.max of the commercial
system voltage e.sub.uw before connecting the power conditioner 1
to the commercial power system 3. If the power conditioner 1 is
connected, immediately after its start-up, to the commercial power
system 3, the commercial system voltage e.sub.uw is suddenly
applied to the capacitor C.sub.f to cause an abrupt increase in the
applied voltage to the capacitor C.sub.f, and therefore, an inrush
current may flow in the capacitor C.sub.f.
[0055] In contrast, according to the present exemplary embodiment,
before connecting itself to the commercial power system 3, the
power conditioner 1 controls to adjust the amplitude E.sub.inv.max
of the inverter output voltage to gradually match the amplitude
E.sub.uw.max of the commercial system voltage e.sub.uw, which
allows the applied voltage to the capacitor C.sub.f to increase
gradually. Therefore, it is possible to eliminate the possibility
that an inrush current may flow in the capacitor C.sub.f when the
power conditioner 1 is connected to the commercial power system 3.
The second feature is to estimate a value of a current
(capacitor-passing current) i.sub.c flowing through the capacitor
C.sub.f, and detect or find a change in an output current i.sub.sp
from the power conditioner 1 before allowing the power conditioner
1 to start grid-connected operation, so as to check a failure
(detect disconnection/welding) of the grid connection relays
S.sub.u, S.sub.w.
[0056] According to the power conditioner 1 of the present
exemplary embodiment, the use of the start-up control method
described above can prevent or reduce an inrush current to the
capacitor C.sub.f provided between the inverter 5 and the
commercial power system 3 at start-up of the power conditioner 1
without using a conventional inrush current prevention circuitry.
In addition, it is possible to reliably detect disconnection and
welding of the grid connection relays S.sub.u, S.sub.w before the
power conditioner 1 is connected to the commercial power system
3.
[0057] In order to confirm the effect of the start-up control
method described above, the grid connection relay S.sub.w was
welded, and an experiment at start-up of the power conditioner 1
was conducted with the grid connection relay S.sub.w welded. Note
that in normal operation, if the grid connection relay S.sub.w is
welded, the operation of the power conditioner 1 is stopped and
prevented from shifting to the grid connected operation as shown in
FIG. 4. However, in this experiment, after welding of the grid
connection relay S.sub.w was detected, the operation of the power
conditioner 1 was intentionally shifted to the grid-connected
operation, and then the occurrence of inrush current to the
capacitor C.sub.f when using the start-up control method was
checked. In short, the purpose of this experiment at start-up of
the power conditioner 1 is to determine, at the same time, the
effect of reducing inrush current to the capacitor C.sub.f and the
effect of checking disconnection/welding of the grid connection
relays S.sub.u, S.sub.w when using the start-up control method
described above.
[0058] FIG. 7 is a graph showing results of this experiment
conducted at start-up of the power conditioner 1 with the grid
connection relay S.sub.w welded. FIG. 7 shows an inverter output
voltage e.sub.inv and a commercial system voltage e.sub.uw in the
power conditioner 1 using the start-up control method described
above immediately before it is connected to the commercial power
system 3, and also shows the occurrence of inrush current to the
capacitor C.sub.f when the grid-connected operation of the power
conditioner 1 is started. In FIG. 7, the one-dot chain curve shows
the inverter output voltage e.sub.inv, and the solid curve shows
the commercial system voltage e.sub.uw, while the dashed curve
shows an output current i.sub.sp from the power conditioner 1. Note
that in FIG. 7, the vertical axis represents voltage (V) and
current (A), while the horizontal axis represents time t (sec).
[0059] According to this experiment, as shown in FIG. 7, the output
current i.sub.sp from the power conditioner 1 became a spike-like
inrush current at the moment when the grid connection relays
S.sub.u, S.sub.w were turned on. Here, the reason for the
generation of the spike-like inrush current is because of a phase
difference .DELTA..phi. (set to 200 .mu.s in this experiment which
corresponds to about 4.32.degree.) given between the inverter
output voltage e.sub.inv and the commercial system voltage
e.sub.uw. Thus, at the moment when the grid connection relays
S.sub.u, S.sub.w are turned on, a difference between the commercial
system voltage e.sub.uw and an AC voltage applied to the capacitor
C.sub.f, which is due to the phase difference .DELTA..phi. between
the inverter output voltage e.sub.inv and the commercial system
voltage e.sub.uw, causes a current to flow in the capacitor
C.sub.f, consequently causing an instantaneous inrush current. Note
here that since the phase difference .DELTA..phi. is set to a small
value as described above, the instantaneous inrush current
generated has a value of up to 20 A. This level of instantaneous
inrush current generated does not exceed a maximum allowable
current of the switching elements SW1 to SW4 of the inverter 5, and
therefore, the entire power conditioner 1 including the inverter 5
operates normally in spite of the inrush current. In addition, in
the experiment, welding of the grid connection relay S.sub.w could
be detected.
[0060] As a result of the experiment, it could be confirmed that it
was possible to reduce an inrush current to the capacitor C.sub.f
by using the method of amplitude and phase adjustment control shown
in FIG. 2 and FIG. 3 to adjust the amplitude and phase of the
output voltage from the inverter 5 at start-up of the power
conditioner 1. In addition, it could be confirmed that it was
possible to detect welding of the grid connection relay by the
disconnection/welding detection process of the grid connection
relay.
[0061] As described above, according to the power conditioner 1 of
the present exemplary embodiment, the amplitude E.sub.inv.max of
the inverter output voltage e.sub.inv from the inverter 5 is
increased stepwise from 0 to adjust the amplitude E.sub.inv/max of
the inverter output voltage e.sub.inv from the inverter 5 to match
the amplitude E.sub.uw.max of the commercial system voltage
e.sub.uw detected by the commercial voltage amplitude detection
circuitry 11 after (immediately after) the start-up of the power
conditioner 1, and thereafter the inverter 5 is connected to the
commercial power system 3 to start the grid-connected operation of
the power conditioner 1. Thus, the voltage applied to the capacitor
C.sub.f provided between the inverter 5 and the commercial power
system 3 can be prevented from abruptly increasing from 0V to the
commercial system voltage e.sub.uw at start-up of the power
conditioner 1. This makes it possible to reduce an inrush current
to the capacitor C.sub.f provided between the inverter 5 and the
commercial power system 3 at start-up of the power conditioner 1
without using an inrush current prevention circuitry.
[0062] Further, according to the power conditioner 1 of the present
exemplary embodiment, the control circuitry 7 (its start-up control
circuitry 13) controls to connect the inverter 5 to the commercial
power system 3 to start the grid-connected operation, after the
phase angle adjustment circuitry 15 adjusts the phase angle of the
inverter output voltage e.sub.inv after start-up of the power
conditioner 1 to allow a phase difference between the phase angle
of the inverter output voltage e.sub.inv and the phase angle of the
commercial system voltage e.sub.uw detected by the PLL circuitry 14
to have a certain value (.DELTA..phi.). Thus, the inverter 5 can be
connected to the commercial power system 3 to start the
grid-connected operation, after not only the amplitude but also the
phase angle of the inverter output voltage e.sub.inv is adjusted by
using the phase angle .theta..sub.uw of the commercial system
voltage e.sub.uw. This makes it possible to further reduce the
change in the voltage applied to the capacitor C.sub.f provided
between the inverter 5 and the commercial power system 3 when the
power conditioner 1 is connected to the commercial power system 3,
and therefore, the inrush current to the capacitor C.sub.f can be
further reduced.
[0063] Further, according to the power conditioner 1 of the present
exemplary embodiment, before it starts the grid-connected
operation, it detects disconnection of the grid connection relays
S.sub.u, S.sub.w based on the value of the output current i.sub.sp
calculated by the system output current value calculation circuitry
20, with the grid connection relays S.sub.u, S.sub.w turned on
(that is, when the grid connection switch is switched to a
connected state), and also detects welding of the grid connection
relays S.sub.u, S.sub.w based on the value of the output current
i.sub.sp calculated by the system output current value calculation
circuitry 20, with the grid connection relays S.sub.u, S.sub.w
turned off (that is, when the grid connection switch is switched to
a disconnected state). Thus, disconnection and welding of the grid
connection relays S.sub.u and S.sub.w can be detected before
starting the grid-connected operation of the power conditioner
1.
MODIFIED EXAMPLES
[0064] It is to be noted that the present invention is not limited
to the above-described exemplary embodiment, and various
modifications are possible within the spirit and scope of the
present invention. Modified examples of the present invention will
be described below.
Modified Example 1
[0065] The above-described exemplary embodiment shows an example
where the claimed grid connection switch is a single grid
connection switch formed by the grid connection relays S.sub.u,
S.sub.w. However, the claimed grid connection switch is not limited
thereto, and can be a dual grid connection switch formed by grid
connection relays S.sub.u.1, S.sub.w.1 and protection relays
S.sub.u.2, S.sub.w.2 as shown in FIG. 8, which is a schematic view
showing, in the left side, a relay circuitry of the grid connection
relays S.sub.u, S.sub.w in the power conditioner 1 according to the
exemplary embodiment, and showing, in the right side, a relay
circuitry of the grid connection relays S.sub.u.1, S.sub.w.1 and
the protection relays S.sub.u.2, S.sub.w.2 in a power conditioner 1
according to Modified Example 1 of the present invention. Note here
that generally, a grid connection relay and a protection relay are
required to be provided in a power conversion device for grid
connection according to the regulations in each country for grid
connection. The protection relay is a relay designed so that when a
failure in, for example, a distributed power supply or a grid fault
occurs, it removes the failure or the grid fault, or limits the
area of the failure or the grid fault.
[0066] Also if the grid connection switch is the dual one formed by
the grid connection relays S.sub.u.1, S.sub.w.1 and the protection
relays S.sub.u.2, S.sub.w.2 as described above, a process similar
to the disconnection/welding detection process in the
above-described exemplary embodiment can be used to detect
disconnection/welding of the protection relays S.sub.u.2, S.sub.w.2
by turning the protection relays S.sub.u.2, S.sub.w.2 on and off in
a procedure similar to FIG. 5, with the grid connection relays
S.sub.u.1, S.sub.w.1 turned on. The process similar to the
disconnection/welding detection process in the above-described
exemplary embodiment can also be used to detect
disconnection/welding of the grid connection relays S.sub.u.1,
S.sub.w.1 by turning the grid connection relays S.sub.u.1,
S.sub.w.1 on and off in a procedure similar to FIG. 5, with both
protection relays S.sub.u.2, S.sub.w.2 turned on.
Modified Example 2
[0067] According to the above-described exemplary embodiment, in
the disconnection/welding detection process of the grid connection
relays S.sub.u, S.sub.w, the passing current value estimation
circuitry 19 calculates a value of a capacitor-passing current
i.sub.c flowing through the capacitor C.sub.f, using the
above-described Equation (5), based on a value of an inverter
output voltage e.sub.inv detected by the output voltage value
detection circuitry 21 so as to estimate the value of the
capacitor-passing current i.sub.c. However, instead, it is also
possible to prepare a LUT (Look-UP Table) associating values of the
inverter output voltage e.sub.inv with values of the
capacitor-passing current i.sub.c, and store the LUT in a memory of
the control circuitry 7, so as to allow the control circuitry 7 to
refer to the LUT to estimate a value of the capacitor-passing
current i.sub.c flowing through the capacitor C.sub.f.
Modified Example 3
[0068] According to the above-described exemplary embodiment, for
detection of whether only one of the grid connection relays
S.sub.u, S.sub.w is welded in periods (2) and (3) in FIG. 5, the
system output current value calculation circuitry 20 repeatedly
calculates the absolute value |i.sub.sp| of the output current
i.sub.sp from the power conditioner 1, with the one of the grid
connection relays S.sub.u, S.sub.w turned off (switched to a
disconnected state), and determines (detects) that welding occurs
in the one of the grid connection relays S.sub.u, S.sub.w, if the
absolute values |i.sub.sp| of the output current i.sub.sp
calculated multiple times in the past are all equal to or more than
a predetermined threshold T.sub.chk, and if such calculated
absolute values |i.sub.sp| continuously increase. However, the
welding detection process is not limited thereto. For example,
similarly as in period (1) in FIG. 5, it is also possible to
directly determine, in periods (2) and (3), that welding occurs in
the one of the grid connection relays S.sub.u, S.sub.w, if an
absolute value |i.sub.sp| of the output current i.sub.sp from the
power conditioner 1 is equal to or more than the predetermined
threshold I.sub.chk.
Modified Example 4
[0069] According to the above-described exemplary embodiment, in
order to detect disconnection and welding of the grid connection
relays S.sub.u, S.sub.w, the control circuitry 7 adjusts the phase
angle of the inverter output voltage e.sub.inv after (immediately
after) start-up of the power conditioner 1 to allow the phase
difference between the phase angle of the inverter output voltage
e.sub.inv and the phase angle of the commercial system voltage
e.sub.uw detected by the PLL circuitry 14 to have a certain value
(.DELTA..phi.). However, the control circuitry 7 can be designed to
allow the start-up control circuitry 13 to control the power
conditioner 1 so that after the phase angle adjustment circuitry 15
adjusts the phase angle of the inverter output voltage e.sub.inv,
after (immediately after) the start-up of the power conditioner 1,
so as to allow the above-described phase difference to have a value
within a predetermined range, the start-up control circuitry 13
connects the inverter 5 to the commercial power system to start the
grid-connected operation. This configuration can also reduce an
inrush current to the capacitor forming the LC filter.
Modified Example 5
[0070] The above-described exemplary embodiment shows an example
where the claimed "capacitor" is the capacitor C.sub.f forming the
LC filter 6. However, the claimed "capacitor" is not limited
thereto, and can be any capacitor connected between the inverter
and the commercial power system (that is, any capacitor on the
output side of the inverter).
Modified Example 6
[0071] The above-described exemplary embodiment shows an example
where the grid connection power conversion device of the present
invention is a power conditioner 1. However, the grid connection
power conversion device to which the present invention is applied
is not limited thereto, and can be a power conversion device for a
hybrid power generation and storage system formed by integrating a
power conversion device, a distributed power supply and a power
storage device.
Modified Example 7
[0072] The above-described exemplary embodiment shows an example
where the control circuitry 7 is formed by a so-called
microcomputer. However, the control circuitry 7 is not limited
thereto, and can be formed by, for example, a system LSI
(Large-Scale Integrated circuit).
[0073] These and other modifications will become obvious, evident
or apparent to those ordinarily skilled in the art, who have read
the description. Accordingly, the appended claims should be
interpreted to cover all modifications and variations which fall
within the spirit and scope of the present invention.
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